Fuel cells are used as a power source for electric vehicles, stationary power supplies and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called MEA (“membrane-electrode-assembly”) comprising a thin, solid polymer membrane-electrolyte having an anode on one face and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, which may contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 and O2/air) over the surfaces of the respective anode and cathode.
PEM fuel cells comprise a plurality of the MEAs stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or current collector. In some types of fuel cells each bipolar plate is comprised of two separate plates that are attached together with a fluid passageway therebetween through which a coolant fluid flows to remove heat from both sides of the MEAs. In other types of fuel cells the bipolar plates include both single plates and attached together plates which are arranged in a repeating pattern with at least one surface of each MEA being cooled by a coolant fluid flowing through the two plate bipolar plates.
The fuel cells are operated in a manner that maintains the MEAs in a humidified state. The level of humidity of the MEAs affects the performance of the fuel cell. Additionally, if an MEA is run too dry, the MEA can be damaged which can cause immediate failure or reduce the useful life of the fuel cell. To avoid drying out the MEAs, the typical fuel cells are operated in a flooded condition wherein the humidity of the MEA is greater than 100% and liquid water is formed in the fuel cell during the production of electricity. Additionally, the cathode and/or anode reactant gases being supplied to the fuel cell are also humidified to prevent the drying of the MEAs in the locations proximate the inlets for the reactant gases.
The operation of the fuel cells with the MEAs humidified greater than 100%, however, limits the performance of the fuel cell stack. Specifically, the formation of liquid water impedes the diffusion of gas to the MEAs, thereby limiting their performance. The liquid water also acts as a flow blockage reducing cell flow and causing even higher fuel cell relative humidity which can lead to unstable fuel cell performance. Additionally, the formation of liquid water within the cell can cause significant damage when the fuel cell is shut down and is exposed to freezing conditions. That is, when the fuel cell is nonoperational and the temperature in the fuel cell drops below freezing, the liquid water therein will freeze and expand, potentially damaging the fuel cell.
Thus, it would be advantageous to control and operate the fuel cell in a manner that prevents and/or limits the formation of liquid water therein. It would be further advantageous if such a control or operation of the fuel cell resulted in the MEA being operated at a humidified state that results in optimum performance.
Controlling the operating conditions within the fuel cell, however, has proved to be difficult. Specifically, the measuring and controlling of the humidity of the gaseous reactant streams flowing into the fuel cell can be difficult. Traditionally, a water vapor transfer device (WVT) is utilized to humidify the cathode reactant gas prior to entering into the fuel cell. The operation of the WVT, however, is difficult to characterize and, as a result, the exact humidity of the cathode reactant gas flowing into the fuel cell may be difficult to ascertain. Thus, it would also be advantageous to control and operate a fuel cell with an improved understanding of the humidity of the cathode reactant gas flowing into the fuel cell.